5. THE NEAR INFRARED BACKGROUND

Following the work of
Hauser et
al. (1998),
there has been significant progress in direct determination of the CIB
in the near infrared window using DIRBE data. Since the uncertainty in
the statistical model of the faint stellar contribution used by
Arendt et al. (1998)
was a major source of uncertainty at these wavelengths, several
approaches have been used to reduce that uncertainty.

Dwek & Arendt
(1998)
assumed that the CIB at 2.2 µm is close to the integrated
light from galaxies at this wavelength. Subtracting the integrated
galaxy light and the zodiacal light
(Kelsall et
al. 1998)
from the DIRBE 2.2 µm maps yielded a map of starlight at 2.2
µm (the ISM contribution at this wavelength is
negligible). Using this 2.2 µm starlight map as a spatial
template for starlight at 3.5 µm, they obtained a
significantly positive 3.5 µm residual,
I = 9.9 +
0.312[I(2.2) -
7.4]±2.9 nW m-2 sr-1, where
I(2.2) is the
actual CIB at 2.2 µm. They tentatively identified this 3.5
µm residual as the CIB, though they did not demonstrate that
it was isotropic. They also obtained somewhat improved upper limits on
the CIB at 1.25 and 4.9 µm using the same stellar template
(Table 1).

Gorjian, Wright,
& Chary (2000)
reduced the stellar foreground uncertainty more directly by measuring
all of the stars brighter than 9th magnitude at 2.2 and 3.5
µm in a dark 2° × 2° field near the north
Galactic pole. They calculated the contribution of fainter stars using
the statistical model of
Wainscoat et
al. (1992).
The uncertainty in the calculated light from objects below this faint
limit is negligibly small, even at high galactic latitude. They also
used a model for the zodiacal light contribution which differed from
that of
Kelsall et
al. (1998).
They argued that the Kelsall et al. model left a high galactic latitude
residual at 25 µm which is dominated by IPD emission. The
IPD model used by Gorjian et al. was similar to that of Kelsall et
al. in that it required that the apparent annual time variation be
reproduced, but it further required that the residual brightness at 25
µm after zodiacal light removal be constant at a value of
zero at high galactic latitude (the "very strong no zodi principle" of
Wright 1997).
After removing the IPD and stellar contributions, Gorjian et al. found
significant positive residuals at 2.2 and 3.5 µm which they
identified as probable detections of the CIB
(Table 1). Had they used the IPD
model of Kelsall et al, their results would have been ~ 40% higher than
those shown in Table 1, a clear
illustration of the uniqueness problem
in modeling the zodiacal light. With a field covering only ~ 8 DIRBE
beams, they did not demonstrate the isotropy of these signals.

Wright & Reese
(2000)
compared the histograms of the pixel intensity distributions in the
DIRBE 2.2 and 3.5 µm maps in five fields at high galactic
and high ecliptic latitudes with the histograms predicted from the star
count model of
Wainscoat et
al. (1992).
The IPD contribution had first been removed from the observations using
the model of
Gorjian et
al. (2000).
They found that the predicted histograms had the same shape as those
observed, but had to be displaced by a constant intensity to agree with
the observed histograms. The necessary shift was consistent in the five
fields analyzed. They interpreted this shift as the CIB, and obtained
average values for the five fields consistent with the values found by
Gorjian et
al. (2000)
(Table 1). They
noted that the histogram method is statistically more powerful for
finding a real residual and less subject to systematic errors in the
star count model than the subtraction approach used by
Arendt et
al. (1998).

Wright (2000)
considerably strengthened the case for detection of the CIB at 2.2
µm. He used the newly released 2MASS catalog to remove the
contribution of Galactic stars brighter than 14th magnitude from the
DIRBE maps at 1.25 and 2.2 µm in 4 dark regions in the North
and South galactic polar caps. Each region was about 2° in
radius. Using the same IPD model as
Gorjian et
al. (2000)
Wright obtained 2.2 µm residuals in his 4 fields consistent
with each other, and consistent with those of Gorjian et al. and
Wright & Reese
(2000).
Hence, there is a strong case for detection of an isotropic CIB at 2.2
µm. The average value of the 2.2 µm CIB
determinations by these three methods is
I =
21.8±5.5 nW m-2 sr-1. The scatter in the 1.25
µm residuals was too large to claim a detection. Wright's
analysis does provide the strongest current upper limit on the CIB at
that wavelength. The dominant uncertainty in all of these analyses of
the near infrared CIB remains the uncertainty in the zodiacal light
contribution.

Kashlinsky &
Odenwald (2000)
found that the amplitude of the fluctuations in the DIRBE 1.25-4.9
µm maps varied with csc(b). Extrapolating to
csc(b) = 0, they found positive intercepts which they attributed
to fluctuations in the CIB. They reported rms fluctuations of
15.5+3.7-7.0,
5.9+1.6-3.7,
2.4+0.5-0.9, and
2.0+0.25-0.5 at 1.25, 2.2, 3.5, and 4.9
µm respectively, where the errors are 92% confidence
limits. Adopting their argument that the fluctuations
are expected to be 5-10% of the CIB, these results are generally
consistent with the direct background determinations listed in
Table 1.